1. Field of the Invention
The present invention relates to an electro-acoustical transducer, and more particularly relates to an electro-acoustical transducer which is capable of realizing a sound reproduction in an ultra-high frequency band.
2. Description of the Background Art
Recently, as a medium such as a DVD and a DVD-AUDIO has become widespread, an electro-acoustical transducer which is capable of reproducing a high frequency band so as to reproduce an ultra-high frequency band sound included in a content of the medium has been desired. In order to realize the reproduction of the ultra-high band sound, electro-acoustical transducers as shown in FIGS. 22A, 22B, 23A, 23B, and 23C have been proposed (e.g., Japanese Laid-Open Patent Publication No. 2001-211497 and the like). FIGS. 22A and 22B are diagrams each showing an exemplary structure of a conventional electro-acoustical transducer. FIG. 22A is a front view, and FIG. 22B is a cross sectional view of the electro-acoustical transducer as cut along a center line AA in a short side direction thereof shown in FIG. 22A. FIGS. 23A, 23B, and 23C are diagrams each showing another exemplary structure of the conventional electro-acoustical transducer. FIG. 23A is a front view of an electro-acoustical transducer. FIG. 23B is a cross sectional view of the electro-acoustical transducer as cut along a center line AA in a long side direction thereof shown in FIG. 23A. FIG. 23C is a cross sectional view of the electro-acoustical transducer as cut along a center line BB in a short side direction thereof shown in FIG. 23A.
As shown in each of FIGS. 22A and 22B, the electro-acoustical transducer includes a yoke 901, a magnet 902, a diaphragm 903, a spacer 904 and coils 905. The yoke 901 is of a concave shape, and is made from a ferromagnetic material such as iron. The magnet 902 is a planar neodymium magnet which is polarized in a thickness direction thereof. The magnet 902 is firmly fixed on an inner bottom surface of the concave portion of the yoke 901, and between the magnet 902 and the yoke 901, magnetic gaps G1 and G2 are formed. A top surface of the magnet 902 and a top surface of the yoke 901 are situated on a common plane, and on the top surfaces thereof, the diaphragm 903 in a film form is firmly fixed via the spacer 904. The coil 905 is patterned on the diaphragm 903 so as to be situated within ranges of the magnetic gaps G1 and G2. At a central part of the magnet 902, a magnetic flux is emitted from the magnet 902 toward a direction substantially perpendicular to a top surface of the magnet 902, on the other hand, at a peripheral portion of the magnet 902, the magnetic flux is emitted toward a direction diagonally to the top surface thereof. The magnetic fluxes then pass through the coil 905. In such static magnetic field, when an electric current flows to the coil 905, a drive force is generated in a direction perpendicular to the diaphragm 903 (an up-down direction in FIG. 22B), and the generated drive force causes the diaphragm 903 to vibrate in the up-down direction, whereby a sound is generated. The drive force is proportional to the magnetic flux, among the magnetic fluxes passing through the coil 905, which is perpendicular to the vibration direction of the diaphragm 903.
In the electro-acoustical transducer, as shown in FIGS. 22A and 22B, a vibrating portion, on which the coil 905 is patterned, is of an elongated shape. Therefore, a resonant frequency of a resonant mode generated in the short side direction of the vibrating portion is high, and a peak/dip is hardly caused by the resonant mode in an ultra-high frequency band. In this manner, in the case of the electro-acoustical transducer shown in FIGS. 22A and 22B, the vibrating portion is formed in the elongated shape, whereby a fluctuation in a sound-pressure frequency characteristic in the ultra-high band, which is caused by the resonant mode, is reduced.
As shown in FIGS. 23A, 23B and 23C, the electro-acoustical transducer includes a frame 906, a yoke 907, a magnet 908, a diaphragm 909, a coil 910 and an edge 911. The frame 906 is of a concave shape. The yoke 907 is of a concave shape and is made from a ferromagnetic material such as iron. The yoke 907 is firmly fixed on an inner bottom surface of the concave portion of the frame 906. On the inner bottom surface of the concave portion of the yoke 907, a magnet 908 of a parallelepiped shape is firmly fixed. The magnet 908 is, for example, a neodymium magnet having an energy product of 44 MGOe, and is polarized in a vibration direction of the diaphragm 909 (an up-down direction in FIG. 23C) As shown in FIG. 23C, due to a structure configured with the yoke 907 and the magnet 908, magnetic gaps G1 and G2 are formed by magnetic fluxes φ at the side of the diaphragm 909. Bold arrows shown in FIG. 23C indicate the magnetic fluxes φ. The diaphragm 909 is of an elongated track shape (hereinafter referred to as elongated track shape), and is situated above the magnet 908. The coil 910 is formed in an elongated ring shape by winding a copper or an aluminum wire several turns, and is bonded on a top surface of the diaphragm 909 with an adhesive agent Ad. Respective long sides of the coil 910 are situated in the magnetic gaps G1 and G2. Specifically, the respective long sides of the coil 910 are situated such that the centers of widths of the long sides of the coil having been wound are located immediately above extremities T1 and T2 of the magnet 908 in the short side direction. Long sides of the magnet 908 and the coil 910 are in parallel with long sides of the diaphragm 909. The edge 911 is of a semicircle shape as viewed in cross section, and an inner-circumference thereof is firmly fixed to an outer-circumference of the diaphragm 909, and an outer-circumference thereof is firmly fixed on a top surface of the frame 906. Accordingly, the diaphragm 909 is supported by the edge 911 such that the diaphragm 909 vibrates in the up-down direction. In the static magnetic field shown in FIG. 23C, when an electric current flows through the coil 910, the drive force is generated in a direction perpendicular to the diaphragm 909 (in the up-down direction in FIG. 23C), and the generated drive force causes the diaphragm 909 to vibrate in the up-down direction, whereby a sound is generated. The drive force is proportional to the magnetic flux, among the magnetic fluxes φ passing through the coil 910, which is perpendicular to the vibration direction of the diaphragm 909.
In the electro-acoustical transducer shown in FIGS. 23A, 23B and 23C, the diaphragm 909 is of the elongated shape as shown in FIG. 23A. Accordingly, as with the electro-acoustical transducer shown in FIGS. 22A and 22B, the resonant frequency of the resonant mode generated in the short side direction of the diaphragm 909 is high, and a peak/dip is hardly caused by the resonant mode in the ultra-high frequency band. In this manner, in the case of the electro-acoustical transducer shown in FIGS. 23A, 23B and 23C, the diaphragm 909 is of the elongated shape, whereby the fluctuation in the sound-pressure frequency characteristic in the ultra-high band, which is caused by the resonant mode, is reduced.
In order to realize a sound reproduction in the ultra-high band in a further improved manner, not only the fluctuation in the sound-pressure frequency characteristic caused by the resonance needs to be reduced, but also a reproduced sound pressure level needs to be improved. In order to improve the reproduced sound pressure level, the drive force generated in the coil needs to be increased, and specifically, the magnetic flux in the direction perpendicular to the vibration direction of the diaphragm needs to be increased. In order to increase the magnetic flux in the direction perpendicular to the vibration direction of the diaphragm, a width of the magnet 902 in the short side direction needs to be increased in the case of the electro-acoustical transducer shown in FIGS. 22A and 22B. In FIG. 22B, the width of the magnet 902 needs to be increased in a left-right direction. In the case of the electro-acoustical transducer shown in FIGS. 23A, 23B and 23C, the width in the short side direction of the magnet 908 needs to be increased. In FIG. 23C, the width of the magnet 908 needs to be increased in the left-right direction.
However, in each of the conventional electro-acoustical transducers shown in FIGS. 22A, 22B, 23A, 23B and 23C, even if the width of the magnet 902 or the magnet 908 is increased, the magnetic flux cannot be efficiently increased in the direction perpendicular to the vibration direction of the diaphragm. Hereinafter, a reason why the magnetic flux cannot be efficiently increased will be exemplified by using the conventional electro-acoustical transducer shown in FIGS. 23A, 23B and 23C.
In the electro-acoustical transducer shown in FIGS. 23A, 23B and 23C, when the width in the short side direction of the magnet 908 is increased, the electro-acoustical transducer will be as shown in FIG. 24. FIG. 24 is a cross sectional view of the electro-acoustical transducer shown in FIGS. 23A, 23B and 23C in the case where the width in the short side direction of the magnet 908 is increased. In FIG. 24, without changing the width in the short side direction of the diaphragm 909, the magnet 908 shown in FIG. 23C is replaced with a magnet 908a, whose width is wider than the magnet 908, and extremities of the magnet 908a in the short side direction are denoted by T3 and T4. The width in the short side of the diaphragm 909 is not changed so as not to cause the sound-pressure frequency characteristic to fluctuate in the ultra-high frequency band. Further, the frame 906 shown in FIG. 23C is replaced with a frame 906a, and the yoke 907 shown in FIG. 23C is replaced with a yoke 907a so as to be adapted to the magnet 908a 
A magnetic flux densities in accordance with a coil position are compared between a case where the magnet 908 shown in FIG. 23C is used and a case where the magnet 908a shown in FIG. 24. A result of the comparison is shown in FIG. 25. As shown in FIG. 25, a vertical axis indicates the magnetic flux density. The magnetic flux density represents a density of the magnetic flux in the direction perpendicular to the vibration direction of the diaphragm 909. The higher the magnetic flux density is, the more the magnetic flux is increased in the direction perpendicular to the vibration direction of the diaphragm 909. A horizontal axis indicates a distance from a central axis O in the short side direction of the diaphragm 909, and a right side of the horizontal axis, that is, the right side of each of FIGS. 23C and 24 indicates a positive direction. In FIG. 25, a graph (a) shows a distribution of the magnetic flux densities in the case where the magnet 908 shown in FIG. 23C is used, whereas a graph (b) shows the distribution of the magnetic flux densities in the case where the magnet 908a shown in FIG. 24 is used.
The graph (a) has a maximum magnetic flux density at a position of each of the extremities T1 and T2. As shown in FIG. 23C, the centers of the widths of the long sides of the coil 910 are located immediately above the extremities T1 and T2, respectively. On the other hand, the graph (b) has a maximum magnetic flux density at a position of each of the extremities T3 and T4. In FIG. 24, in order not to cause the sound-pressure frequency characteristic to fluctuate in the ultra-high frequency band, the width of the diaphragm 909 in the short side direction is not changed. That is, the long sides of the coil 910 shown in FIG. 24 are located at the same positions as the long sides thereof shown in FIG. 23C, respectively, and thus are situated immediately above the positions of the extremities T1 and T2. Therefore, in the graph (b), the magnetic flux density at a position where the coil 910 shown in FIG. 24 is situated is increased only by δB compared to the magnetic flux density at the same position in the graph (a).
In this manner, in the conventional electro-acoustical transducers shown in FIGS. 22A, 22B, 23A, 23B and 23C, even if the widths of the magnets 902 and 907 are increased, the magnetic fluxes in the direction perpendicular to the vibration direction of the diaphragm cannot be increased efficiently. Accordingly, it is difficult, in the conventional electro-acoustical transducers shown in FIGS. 22A, 22B, 23A, 23B and 23C, to realize the sound reproduction in the ultra-high frequency band efficiently.